1. Field of the Invention
The present invention is directed generally to communication cabling for transmitting signals, and more particularly to reduction of crosstalk between those signals.
2. Description of the Related Art
Communication cabling typically contains multiple wires dedicated to different circuits, communication channels, or devices. For instance, a communication cable can have multiple pairs of wires (such as pairs of copper wires) each pair being used for different communication functions. A conventional communication cable, includes four twisted-wire pairs (also known as “twisted pairs”). Each of the wires in the twisted pairs is substantially identical to one another. As is appreciated by those of ordinary skill in the art, the wires each include an electrical conductor (e.g., a conventional copper wire) surrounded by an outer layer of insulation (e.g., a conventional insulating flexible plastic jacket).
Each of the twisted pairs serves as a differential signaling pair wherein signals are transmitted thereupon and expressed as a specific ratio of balanced voltage differences and balanced current differences between the wires of the twisted pair. A twisted pair can be susceptible to electromagnetic sources including another nearby cable of similar construction. Signals received by the twisted pair from such electromagnetic sources external to the cable's jacket are referred to as “alien crosstalk.” The twisted pair can also receive signals from one or more wires of the three other twisted pairs within the cable's jacket, which is referred to as “local crosstalk” or “internal crosstalk.”
As signal frequency increases, the individual signals tend to increasingly interfere with one another (i.e., local crosstalk increases) due to the close proximity of the wire pairs. While twisting the two wires of each pair together helps considerably to reduce crosstalk, it is not sufficient by itself as signal frequency increases. Conventional approaches such as using physical spacing within the cable to physically separate and isolate the individual twisted wire pairs from one another can be also help reduce crosstalk. However, the additional physical spacing increases cable diameter and decreases cable flexibility. Other conventional approaches of reducing local crosstalk occurring between these wire pairs include wrapping the wire pairs with metal foil or wire braid, which unfortunately can involve additional assembly, material costs, and cable stiffness.
FIG. 12 depicts an exemplary conventional approach of reducing local crosstalk in a shielded twisted pair cable 10. The prior art shielded twisted pair cable 10 has four twisted wire pairs or twisted pairs 104, 106, 108, and 110 covered by an internal sheath 12. As may best be seen in FIG. 13, the first twisted pair 104 includes wires 104a and 104b, the second twisted pair 106 includes wires 106a and 106b, the third twisted pair 108 includes wires 108a and 108b, and the fourth twisted pair 110 includes wires 110a and 110b. The wires of each of the twisted pairs 104, 106, 108, and 110 are twisted together in accordance with a different twist rate. Thus, each of the twists of the wires 104a and 104b of the twisted pair 104 has a first twist length TL-1, each of the twists of the wires 106a and 106b of the twisted pair 106 has a second twist length TL-2, each of the twists of the wires 108a and 108b of the twisted pair 108 has a third twist length TL-3, and each of the twists of the wires 110a and 110b of the twisted pair 110 has a fourth twist length TL-4.
Returning to FIG. 12, in particular implementations, the twisted pairs 104, 106, 108, and 110 may be twisted together in a bundle (not shown) under the internal sheath 12. The bundle has a twist period that can be, and typically is, longer (i.e., has a lower twist rate) than the twist periods of the twisted pairs 104, 106, 108, and 110. The internal sheath 12 may be covered by insulation 14 (such as Mylar), which is covered by a conductive shield 16. The conductive shield 16 can be used to a certain degree to reduce crosstalk by reducing electrostatic and magnetic coupling between twisted wire pairs contained within the internal sheath 12.
A drain wire 18 is electrically coupled to the conductive shield 16. An external sheath 22 covers the conductive shield 16 and the drain wire 18. The conductive shield 16 is typically connected to a connector shell (not shown), which grounds the conductive shield 16, on each cable end usually through use of the drain wire 18. Connecting the conductive shield 16 to the connector shell can be problematic due to additional complexity of installation, added cable stiffness, special connectors required, and the necessity for an electrical ground available at both ends of the cable 10. Furthermore, improper connection of the conductive shield 16 can reduce or eliminate the effectiveness of the conductive shield and also can raise safety issues due to improper grounding of the drain wire 18. In some improper installations, the conventional continuous shielding of a cable segment is not connected on one or both ends. Unconnected ends of conventional shielding can give rise to undesired resonances related to the un-terminated shield length which enhances undesired external interference and crosstalk at those resonant frequencies
Thus, unfortunately, crosstalk remains a problem particularly in communications cables carrying signals having higher frequencies. Therefore, a need exists for communication cables configured to reduce alien crosstalk and/or local crosstalk. The present application provides these and other advantages as will be apparent from the following detailed description and accompanying figures.